How long does it take for mortar to show final color?

Hello, my contractor mortared our flagstone firepit 48 hours ago and the mortar color is really dark (dark dark grey, close to black). It's been 48 hours and the color has not lightened up much. It's a STARK contrast to our light gold/silver flagstones. We asked him to use a light grey mortar and we are not sure if the color is due to the fact that its wet or the fact that he used the wrong color. When does mortar show it's final dry color? Thanks!